I prefer Google as the results seem more relevant to the typed query. When a search query is entered, Yahoo results seem to be more like text matching while Google results seem to be concept matching which returns more relevant results.
The primary reason I use Google is that they support searching good'old usenet. Today they mix these with GoogleGroups, but even so, it can be a wonder to search through decades of posts to find a solution to some annoying and rare problem.
I have found yahoo to return more quality sites where google returns more quantity. I do like the define: function of google though. I use it often for acronym and general research. I have yet to succeed in getting one of my glossaries to show though.